Previously, we have demonstrated with freeze-fracture electron microscopy that the belts of endothelial tight junctions, which impede diffusion between blood and brain, were reduced to fragmentary, small junctions in vitro. However, when astrocytes were co-cultured with the brain endothelial cells, the endothelial tight junctions were enhanced in length, width, frequency and complexity. We now attempt to measure the functional permeability of these brain endothelial cells in vitro. The electrical resistances across the confluent cell layers were consistently higher in the brain endothelial cells co-cultured with astrocytes, than without. Thus, astrocytes appear to influence the brain endothelial cells' permeability to ions in vitro.